1. In-house MAPREC tests of all OPV lots submitted to CBER for release were performed. 2. Reference viral samples for type-3 OPV for molecular assay were prepared in bulk amounts and tested against DNA reference materials as part of the WHO Collaborative Study of MAPREC led and coordinated by LMD. 3. Reference viruses were sent to NIBSC, UK, for ampouling and stability testing. 4. An accelerated degradation study was performed in collaboration with NIBSC, UK, to determine stability of provisional International Reference DNA Reagents for the MAPREC Study. 5. The next Phase of the WHO Collaborative Study on MAPREC with type-3 OPV was initiated. All necessary manuals, documentation and other materials were prepared and shipped to 15 participants. Progress of the Study was monitored, and troubleshooting and other help provided when requested by the participants. 6. A study of type-3 OPV neurovirulence determinants was performed. Four commercial strains of Sabin type-3 poliovirus and two wild-type progenitor strains were sequenced, and their interrelations established. New mutations that may affect attenuation of the virus were found. 7. A new method for genetic micro-manipulations with poliovirus genome was developed and used to prepare site-directed viral constructs of type-1 and type-2 OPV. 8. Collaboration with OPV manufacturers to introduce MAPREC for evaluation of acceptability of OPV was continued. Efforts were concentrated on a study of developmental lots of type-2 OPV produced in a new cell substrate--lots that had failed the monkey neurovirulence test. The project will be continued. Three papers were published.